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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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24th Mar 2023, 7:20 pm | #1 |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Low cost DC-DC boost converter 400v
I bought one of those very low cost variable DC-DC boost converters with the idea of maybe using it in a valve tester. I bought the -400v 0 +400v version.
I thought I'd try it out on a pre-amp, the pre amp worked well but unless the voltage is set to minimum or maximum it has a nasty 10Hz sawtooth wave on top the DC, about 1vp-p. Any ideas about filtering it? I could set the output to max and use some extra RC filtering with dropping resisters to set the voltage. Doug |
24th Mar 2023, 7:42 pm | #2 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
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Re: Low cost DC-DC boost converter 400v
I'm probably not going to be able to suggest a simple fix for this. But other people might, in which case I suspect that they'll want to know how much current this device is going to be delivering. What current is the device rated for ?
Cheers, GJ
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24th Mar 2023, 7:50 pm | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: Low cost DC-DC boost converter 400v
Try a bit of load on it and see if it goes stable. It looks like a feedback loop just hunting off load.
David
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24th Mar 2023, 7:58 pm | #4 |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Re: Low cost DC-DC boost converter 400v
I'll give it some work to do, maybe try some 15W bulbs from scrapped microwave ovens.
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24th Mar 2023, 8:00 pm | #5 |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Re: Low cost DC-DC boost converter 400v
Yes, defo looks like the feedback loop, at about 50v output the waveform is visible but not enough to trigger the small scope, about 65v it is able to measure it.
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24th Mar 2023, 8:38 pm | #6 |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Re: Low cost DC-DC boost converter 400v
I'm not expecting to power much as they are about £7 and quite small, 60mmx50mmx20mm.
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24th Mar 2023, 8:43 pm | #7 |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Re: Low cost DC-DC boost converter 400v
Photo added
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24th Mar 2023, 10:49 pm | #8 | |
Octode
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,654
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Re: Low cost DC-DC boost converter 400v
Quote:
Richard |
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24th Mar 2023, 11:30 pm | #9 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,679
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Re: Low cost DC-DC boost converter 400v
If the ripple is at 10Hz, it's definitely going to be the converter's feedback loop being unstable. Adding capacitance is unlikely to fix it. It'll just slow it down. Adding a bit of load may well help, as has been suggested above.
That said, boost (and by extension, flyback, as used in very many small mains switch-mode power supplies) converters are notoriously difficult to get to be stable under varying load conditions. This doesn't usually matter when they're powering a more or less constant load that isn't too fussy about ripple and noise. Data sheets for (good) boost converter chips will typically go in to some detail about the frequency compensation arrangements needed in the feedback loop to keep them stable. The fundamental problem is that trying to increase the duty cycle of a boost converter because the output voltage is low actually reduces the output voltage for a few cycles, because the decreased off-time of the switch allows less time for the energy in the inductor's magnetic field to flow out in to the load. The output power only starts to increase once a new steady state is found. This delayed-action effect plays havoc with the feedback loop's attempts to keep a constant output under varying load. Chris
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What's going on in the workshop? http://martin-jones.com/ Last edited by cmjones01; 24th Mar 2023 at 11:36 pm. |
24th Mar 2023, 11:59 pm | #10 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 901
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Re: Low cost DC-DC boost converter 400v
Have you tried alternative input supply and local buffer cap. A weak supply may be another cause of what is being observed. I will see if I have a schematic and notes, as that PCB looks similar to what I tested a few years ago ( but populated for just single output).
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25th Mar 2023, 2:38 pm | #11 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 901
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Re: Low cost DC-DC boost converter 400v
I had a look at that pcb back in 2016 - the link below includes a schematic. I haven't used that smps since then as it had pulse skipping and burst type modes depending on the loading.
https://www.diyaudio.com/community/t...ol-ics.297112/ Since then I have found the 12V dc-ac inverter modules much more convenient, as they just pump out power without any quirky operating modes, but do need a rectifier and filter added to an appropriate secondary tapping. Ebay typically has them with a header description something like "DC 12V to AC 110V 220V 150W Inverter Boost Transformer Power Adapter" - they can come in different layouts, and are based on the SG3525A control IC. Some links that may be relevant: https://www.diyaudio.com/community/t...-for-b.313234/ https://www.aggh.net/discussion/index.php?topic=51615.0 Last edited by trobbins; 25th Mar 2023 at 2:43 pm. |
25th Mar 2023, 3:03 pm | #12 | |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Re: Low cost DC-DC boost converter 400v
Quote:
Doug |
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26th Mar 2023, 1:42 am | #13 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 901
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Re: Low cost DC-DC boost converter 400v
Doug, a recent restoration of just a Ferrograph 2A/N amp chassis from 1955 needed a new B+ and heater supply as they weren't on the original amp chassis. I used an external 12Vdc 5A plugpak and retrofitted a 12V dc to ac inverter pcb (set up for 300Vdc B+) on the amp chassis - quite simple and convenient as the amp chassis had plenty of spare room. Ciao, Tim
https://dalmura.com.au/static/Ferrog...2A-N%20amp.pdf Last edited by trobbins; 26th Mar 2023 at 2:10 am. |
26th Mar 2023, 9:43 am | #14 | |
Hexode
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 349
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Re: Low cost DC-DC boost converter 400v
Quote:
Is this the one, photos attached, Looks like couple of MOSFETS buzzing 20KHz into transformer primary with various taps on the secondary. Only £6.50 so bought one to try, while I wait for it to arrive I might crack open an old car cigarette lighter adapter 12v-230v rated for 150W that used to power a laptop on the move from the late 1990s to find out what's inside. Do you know if the pad at V1 is connected to a tap on the transformer? The default output voltage wiring as follows: V0-V2 = 110 V V0-V3 = 172 V V0-V4 = 220 V V0-V5 = 220 V V5-V4 = 20 V V5-V3 = 45 V V5-V2 = 110 V V4-V3 = 28 V V4-V2 = 90 V V3-V2 = 62 V Doug |
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26th Mar 2023, 11:17 am | #15 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 901
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Re: Low cost DC-DC boost converter 400v
Yup, that's the 'long' version I bought a few of. Photo of the rectifier & filter I used for the Ferrograph amp. V1 pin has no winding wire connected to it, but I snipped the pin off anyway as I used the V1 pad for the rectifier/filter wiring - which uses smt diodes and enough cap (100nF) for local smoothing of the switching frequency.
Because I could, I added a smt cap to bypass 3525A Vcc and Vc direct to 0V trace going to main cap. I added an smt cap directly across the main 2200uF cap pads and bolstered the gnd plane between FET sources. I sat the FET tabs flat and bolted to pcb to improve thermal conduction and lower resistance to transformer terminals - as this application didn't need additional heatsinking. Timing resistor RT on 3525A pin 6 is 4.3kohm to 0V, to provide 25kHz switching frequency with timing cap CT of 5N6. RT can go down to 2k. I changed RT to 3k4. As well as the 3525A app notes, some other forum threads that have some insight are: https://www.diyaudio.com/community/t...-for-b.313234/ https://www.diyaudio.com/community/t...-37khz.311812/ |
27th Mar 2023, 6:39 pm | #16 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Low cost DC-DC boost converter 400v
I've used one of those converters to replace the vibrator PSU [whose transformer had failed] in a R209 receiver.
See this thread: https://www.vintage-radio.net/forum/....php?p=1503469 Yes, they are intrinsically noisy, but with a fistful of small chokes and feedthrough-capacitors, along with some decent shielding [in my case a soldered-together box made of PCB material], they can be made quiet-enough to live inside the same case as a HF receiver. I never did fully scope/analyse the waveforms involved, but I suspect they use several methods [both classic PWM and some sort of mode where they skip oscillation-cycles if the output voltage is already high enough] to do the required regulation.
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